One of the principal insulation materials for conductors used in telephone communication systems is a pulpous material which is made by converting sheets of dry wood pulp to a pulp and water slurry and which has acceptable dielectric properties at voice frequencies. Pulp insulation, because of the inherent nature of the pulpous material tends to absorb moisture to which a cable may be exposed, thereby avoiding degradation of transmission signals. Water which enters a pulp-insulated multipair cable causes the pulp to swell, which localizes the water at a fault point. Routine tests based on electrical discontinuities caused by the wet pulp are then employed to accurately locate the fault.
Pulp-insulated cable advantageously is low in cost, has a fail safe nature which allows group location of defects caused by failures in cable jackets and provides more conductors for a given cable diameter than other kinds of cable. Another advantage in today's world is the continuing availability of wood pulp as opposed to the increasing dependence on sources for a petroleum derivative from which plastic insulating materials are made.
Pulp-insulated conductors are generally produced in a continuous process in which many conductors, often as many as sixty, are passed through an electrolytic cleaner, coated with a wet pulp layer, and drawn through a drying oven in order to produce a pulp-insulation cover having a final moisture content in the range of 7 to 10% on each of the conductors. A detailed description of a pulp-insulating process can be had by referring, for example, to an article "Manufacturing Pulp Cable", on pages 86-94 of the July-October 1971 issue of The Western Electric Engineer.
One of the problems in pulp-insulated conductors is the occurrence of uninsulated areas along the conductors, particularly in the final cable structure. Uninsulated portions of 0.32 cm or less are called "chips"; 0.95 cm or less, "shorts"; and 0.95 cm or greater, "bare wire". These may occur either because of a lack of adherence of the pulp to the conductor during insulating or because of the abuse to which the insulation is subjected in steps of a cable-making process subsequent to insulating. Conductors which have a predetermined number of such defects occupy a part of and increase the size of the cable cross-section without contributing to its utility since they are unuseable for telecommunications. Unfortunately, an increased cable cross-section requires additional plastic jacketing material and underground duct capacity without any offsetting benefit.
Another problem relates to the strength characteristics of pulp insulation and the effects of these on the electrical properties of pulp-insulated conductors. A pulp-insulated conductor emerging from a final drying chamber has a substantially circular cross-sectional configuration. When two such insulated conductors are associated together to form a twisted pair, their centers are separated by a distance which has an inversely proportional effect on mutual capacitance. Because the crush resistance or compressive strength of conventional pulp-insulation having a residual moisture content is relatively low, one or both of the conductors may have its insulation deformed when subjected to the rigors of other manufacturing processes such as, for example, twisting. This generally causes the distance between conductor centers to be decreased with an accompanying undesirable increase in mutual capacitance. While this problem could be overcome by reducing the residual moisture content, the resulting pulp insulation has been found to have unacceptable flexibility endurance characteristics.
If the foregoing problems were to be overcome, pulp, because of its advantages, could possibly become more attractive for conductor insulating in the telecommunications industry. Moreover, if the foregoing problems were to be overcome, savings could be realized on jacketing material and duct space since an extremely high percentage of the conductors in a cable would be usable, thereby obviating the need to provide spare pairs to guarantee a predetermined number of usable ones.
In the prior art, C. J. Krogel, in U.S. Pat. No. 2,440,802, shows a method in which a wire is coated with a water emulsion of polyvinyl acetate, polyvinyl alcohol or methyl methacrylate and moved through a vat of water-pulp slurry. See also U.S. Pat. No. 1,615,422, issued to H. G. Walker et al.
What the prior art seemingly fails to show or teach is a suitable process for pulp-insulating conductors which insures consistent integrity of the insulative covering on the conductor. Such a process should permit the pulp to be treated in such a way as to optimize its resistance to deformation in subsequent processing and during installation to preserve acceptable mutual capacitance properties, without causing the pulp to become brittle and impair the integrity of the insulative cover.